As a communication traffic in a backbone optical network is increased, a transmission capacity of an optical communication system is expanding. As an example of a technology available to expand the transmission capacity, a WDM technology in which plural wavelengths are wavelength-division-multiplexed has been used in the optical communication system.
By using the WDM technology, a plurality of wavelengths (in other words, multi-channels) can be transmitted in a single optical transmission. Thus, it is possible to achieve large-capacity (in other words, high-speed) optical communication.
A coherent optical receiver is applicable as an optical receiver that receives light transmitted with the multi-channels. Even when light with the multi-channels is input to the coherent optical receiver, the coherent optical receiver is available to selectively receive a target channel light having a wavelength desired to be received by performing a coherent detection.